By the co-inventors of the subject invention a method was developed some time ago for producing a supported catalyst for polymerization of ethylene and copolymerization of ethylene and .alpha.-olefin, by following the way of coating the catalysts with a transition metal compound, that is, first reacting an organomagnesium compound having the structure of MgPh.sub.2.nMgCl.sub.2.mR.sub.2 O (here, n=0.37.about.0.7; m.gtoreq.2; R.sub.2 O=ether; Ph=phenyl) with an organic halide, and next coating the thus produced carrier with such transition metal compounds as TiCl.sub.4, VCl.sub.4, or VOCl.sub.3 (Japanese Patent Application No. 330675/1995).
The catalyst produced by this publicly known art, particularly the catalyst produced by coating a carrier with VCl.sub.4 marked an advance in part of the polymerization processes by producing polymers of a narrow particle size distribution and an increased bulk density, yet it is found to accompany such problems as yielding polymers of an uncontrolling broad molecular weight distribution and a gradual inactivation of the active substance of vanadium during polymerization.
Meanwhile, a process has been known for producing a catalyst by first reacting magnesium-aluminum-alkyl compound (RMgR.sub.1 -nAlR.sub.3 -mD) and hydrocarbon chloride, and then reacting the thus obtained solid product (carrier) with a titanium or a vanadium halide (German Patent Application No. 3636060: French Patent Application No. 2529207). In this process, (n-Bu)Mg(i-Bu) or (n-Bu)Mg(OCl) dissolved in hydrocarbon is used as an organomagnesium compound, RMgR', and tert-BuCl is used as hydrocarbon chloride. The main defect of this process is found in the insufficient activity of the produced catalyst.
EP Application EP-A-0, 155,770 is publicly known to teach the production of a catalyst by precipitating a vanadium compound on an elliptical support of magnesium chloride including a composition which contains an electron-donating compound. This catalyst is used in production of ethylene polymers of a wide molecular weight distribution. This catalyst, however, has a defect in that although only a little quantity of vanadium compound is fixed on the support, a relatively large quantity of vanadium compound is required for the purpose. In the performance of washing the catalyst it is generally required to remove the surplus vanadium compound which has not been fixed on the support, and this removal costs much money and is troublesome, because of its toxicity and corrosive properties.
It is publicly known that a polyethylene resin requires a proper molecular weight distribution respectively for different production processes.
Hence, it is required either to adopt a multiple-stage of reactors to produce polyethylenes of different molecular weights from one another from the respective reactors, for control of molecular weight distributions, or alternatively, to develop, by the use of a particular catalyst-activating substance, a catalyst capable of controlling polymers' molecular weight distributions. A multiple-stage of reactors will naturally require a heavy initial investment of capital, and therefore it is preferable to secure a catalyst which can control polymers' molecular weight distributions.